This invention relates to the field of electrical engineering, and particularly to electrical controls or scanners for use in industrial systems. For the purpose of this application, a scanner may be defined as apparatus for initiating a control function in accordance with a light signal determined by a condition being observed or monitored. Simple examples of such conditions are the presence of a web in a gap, the presence of a registration mark on such a web, the continuous flow past a counting site of objects to be counted, and the movement of an object to be sprayed into a spraying zone.
Industrial applications of scanners have an environment of vibration and frequently of atmospheric moisture or inflammable or explosive vapors. Such scanners must be reliable over long periods of operation, must be capable of ready adjustment as to sensitivity and as to operational delay and hold times, and must be designed for rapid servicing when such servicing is needed, to avoid extended down-time of an industrial line. It is highly desirable that such devices also be so arranged that changes in the use of the line can be accommodated by substitution of interchangeable portions of the control, rather than by rewiring or removal of one control and substitution of another--a more lengthy and expensive process.